In the fabrication of molded parts from thermoplastic material containing a blowing agent, it is well-known to inject a measured amount of the plasticized plastic into the cavity of a mold, the amount of plastic injected being smaller than the volume of the cavity.
Since inside the mold cavity only the pressure of the escaping air normally works against the injection of the plastic, the finely-distributed foaming agent in the plasticized plastic mass can expand which causes an apparent volume enlargement of the plasticized plastic and with it the result is a complete filling of the mold cavity.
Considering that the temperature of the mold is below the temperature of the injected plastic, a temperature difference occurs between the core and the surface of the molded part which (together with the pressure of the foaming agent acting inside the core) causes the forming of the outer skin of the molded part. The outer skin has a correspondingly rough outer skin which has to be worked and ground before any final lacquer can be applied, which adds to a considerable price increase for fabrication of molded parts.
A process for the fabrication of molded parts from a thermoplastic mass containing a foaming agent is taught in German Pat. No. 1,504,278, wherein the plasticized mass is pressed directly against an adjustable gas cushion after exiting from the jet of an extruder, the gas escaping under the opposing pressure of the incoming plastic mass. By using this procedure, the surface of the molded part can be improved, but before final finishing usually one or two working steps are necessary. The density of such molded parts is, however, still correspondingly high.
To improve the surface of molded parts made thermoplastic masses containing foaming agent, a process is taught by German Pat. No. 1,181,897, in which, under steam pressure of the foaming agent, an exceeding pressure heats up to the foaming temperature of the plastic mass and under constant pressure the mass is injected into the mold, where its volume is smaller than the volume of the finished molded part. After the mass under pressure is cooled on the surface and forms a smooth surface, the pressure is eliminated by expansion of the mold cavity, so than the inner mass (which is not cooled off) may further expand. The cooled solid surface layer remains on the wall of the enlarged mold and a foamed core is formed. Because of the expansion of the mold, it is quite expensive. This expenditure is still increased when, during injection of the plastic mass, operations have to be performed against gas pressure, since additional tightness of the mold is required. Additionally, molded parts fabricated in a expandible mold have a so-called "breathing" mark, which is generally considered undesirable.
U.S. Pat. No. 3,384,691 shows a procedure for fabrication of plastic molded parts with a foaming core and a non-foamed outer skin, in which a plasticized mass with forming agent is injected into a mold. The volume of the injected mass is equal to the mold volume, so that foaming cannot occur. After a predetermined time necessary to form a non-foamed outer surface, a part of a still plasticized mass is withdrawn from the core of the mold part through an injection canal into an auxiliary storage container designed as a piston-cylinder unit, which provides consequently an enlargement of the mold volume and corresponds to the "breathing" or expansion of a mold. The plastic mass which contains the blowing agent can now foam; it is still plasticized in the core of the molded part and lies within the auxiliary storage container. This known procedure, however, has the disadvantage that the mass which has been withdrawn into the auxiliary container and which foams after the enlargement of the mold volume, cannot be used again for the next injection procedure, since the surface then has streaks. The withdrawn plastic mass has to be considered as waste. This waste is separated from the molded part after its final solidification and after the opening of the mold. With these and other objects in view, as will be apparent to those skilled in the art, the invention resides in the combination of parts set forth in the specification and covered by the claims appended hereto.